A standard rack drive comprises a stationary part and a movable part displaceable along the stationary part. Normally the stationary part is formed with an upwardly open channel in which a rack is fixed, the rack being exposed upward through the slot of the channel. The movable part can travel along the stationary part and is provided with a drive gear engaging down through the slot with the rack.
In a standard arrangement an elongated toothed belt has a stretch that lies atop the channel over its slot, and is threaded through the movable part, passing under two outer and lower idler rollers and over a toothed wheel fixed on the shaft of the drive gear between and above the outer rollers. The belt therefore serves the function of covering the slot and thereby keeping the rack clean to both sides of the movable part, and of rotating the drive gear. Unfortunately such an arrangement cannot quite produce the accuracy of movement necessary for instance for machine-tool applications, as the long belt needed will inherently have some elasticity. In addition such long toothed belts are extremely difficult to manufacture, so the resultant equipment is quite expensive.
German patent No. 1,101,096 proposes a system for moving a boring quill in the head or tailstock of a lathe. A belt covers the rack and passes in a loop over a deflecting idler roller in the headstock. Beneath the deflecting roller is the drive gear which is independent of this roller. The range of motion here is relatively short. In addition converting a standard toothed-belt system to this type of arrangement requires replacing the belt with a longer one and generally rebuilding the entire device.